1. Field of the Invention
This invention relates to flexible shaft couplings particularly to the type having an annularly discontinuous elastomeric portion joining a pair of shaft mounted hubs.
2. Description of the Prior Art
Flexible shaft couplings are used to physically and functionally connect the ends of two shafts positioned in end-to-end relation by bolting through holes formed in radially extending flanges and correspondingly aligned holes through the legs of angle shaped metal shoes, each located at the axial ends of the coupling. These shoes are formed to shape; placed in a heated mold into which is poured liquid, uncured elastomeric material; and bonded to the flexible material as the elastomer cures in the presence of heat. To facilitate assembly and disassembly, typically the couplings are formed in two annular halves. This construction requires the shoes and flexible center elements to be discontinuous around the circumference and, as is usually the case, each to terminate on a common radial plane.
A common, recurring problem in the use of couplings of this type is the tendancy toward progressive and premature failure of the elastomer particularily near the bondline at the shoes. Flexible couplings, for the most part, function by transmitting primarily torque load from one shaft to another. Bending and axial loads are usually applied at low levels or are reduced to minimal magnitudes as a result of the flexible nature of the associate load paths through the coupling. The coupling offers, for example, very little resistance to bending about an axis normal to the shaft since the cross-section of the center element is extremely and intentionally inefficient in this respect. Static equilibrium requirements for structures consisting of a system of shafts to which bending loads are applied would necessitate that internal bending moments approach zero at the coupling since the bending stiffness of the coupling is so minimal as to stimulate a pinned connection between shafts in the flexural sense. Likewise, axial forces cannot rise to excessive magnitudes as a result of the slight axial displacement of one shaft with respect to another that usually occur, because the coupling, unlike the shafts it joins, is particularly flexible in the axial sense. Large axial motions would be required to develop substantial forces in this direction but are unlikely to occur in practical shaft systems. Where they do occur and flexible couplings are present, the elastic properties of the coupling accomodate sizable displacements longitudinally without correspondingly large internal axial forces being produced.
Torsion is transferred from the metal shoe at one coupling end to the opposite shoe through the center element principally by shearing radially through the elastomer. At the circumferential free edge of the center element where one coupling half abuts the mating second half, the internal shear stresses in the elastomer rise to a peak value that is greatly in excess of the average shear stress existing at points distant from the free edge. This local phenomenon has been particularly troublesome in applications of flexible couplings to systems where the torsion loads contain, in addition to, or perhaps instead of, a steadily applied torque component, cyclical, vibratory or shock components of torque. Occurrences of this sort are common in mechanical equipment. Starting and stopping conditions frequently produce shock or impulse loadings that are large in relation to the rated load of the coupling. In normal operation reciprocating equipment applies continual vibratory loads to the shafts that carry power to or from the equipment.
Cyclic and shock loading contribute to a spectrum of vibratory forces that combine to produce failure of the parts to which they are applied at load levels considerably below that which they could carry if the loads were static in nature. The observation of failures in this cyclic environment is well documented and is commonly referred to as fatigue.
Flexible couplings, then, are subject to local peaking shear stresses at the circumferential extremities of their mating halves and the load environment in which they operate is frequently fatigue-inducing. Designing couplings capable of satisfying service life requirements, particularly as regards premature fatigue shear stress failures, has been a troublesome problem of long standing.